CIVIL 3811 Tutorial Solution - Week 7 - The University of Sydney PDF

The University of Sydney School of Civil Engineering CIVIL 3811/8811/9811 – Tutorial Solution Week 7 (A Possible Solution) Bridge Redesign Design Brief This report will consider the redesign of a 55m long vehicular bridge, with the original width retained. The bridge...

The University of Sydney School of Civil Engineering CIVIL 3811/8811/9811 – Tutorial Solution Week 7 (A Possible Solution) Bridge Redesign Design Brief This report will consider the redesign of a 55m long vehicular bridge, with the original width retained. The bridge will use the original end abutments and piers. The bridge must maintain a 5.7m clearance over a highway. The existing bridge can only be out of use for 1 week in April and December. The site is a rural region, and the client expects the bridge to be aesthetic. Assumptions 1. The substructural foundation has 4 supports on weak rock. The combined load (loads discussed in points 5 & 6) on each foundation does not exceed 1MPa. 2. The soil/weak rock is sufficiently consolidated and incompressible. No further settlement (especially differential settlement) takes place. 3. Short- and long-term ultimate limit states (ULS) in strength, and serviceability limit states (SLS) are considered according to AS5100:2017. It is assumed to satisfy. 4. The bridge deflection does not exceed limit from the Australian Standards. 5. The bridge can withstand seismic and wind loading. Wind is largely laminar (hence uniform, although design accounts for turbulence for further safety) in rural regions due to vast open space. 6. Transient loads are within the capacity of the bridge. 7. The natural frequency of the bridge is sufficiently different to the vibrations imposed by transient loads, such that resonance does not take place. Design Inspiration for the redesigned bridge was obtained from the Juscelino Kubitschek Bridge in Brasilia, Brazil. The bridge designed in this project will be a concrete through arch bridge. This is suitable for spans 35m – 200m long (De Los Santos 2021). Arc bridges are commonly iconic and given the lack of vertical heigh restraints in rural areas, this will be suitable. For further aesthetic, the arc will be adjacent to the bridge deck, and will asymmetrically span to the other side of the deck at the opposite end. The arch will make use of the existing pier foundation into the ground. As it is an arch bridge, although the span is large, the segment lengths are small. This is the distance between the cable restraining the bridge deck. So, the bridge doesn’t have to be as stiff, and can be lighter than previously. Hence with the arch, the bridge will be the same weight as before, and does not exceed the bearing capacity. Ends of the bridge deck will use the existing end foundation. Here, bridge bearings are used to safely transfer the loads to the foundation. Dampeners are also used to minimise the effects of earthquake loadings and resonance due to vibrations. The parapet walls will be designed to contain reinforcement. A bridge height of 5.7m, although may not be substantial, it is over a busy highway. Hence falls will be more fatal. The reinforcement is presumed to absorb energy (without cracking) in the event of an accident. A barrier is also provided between the road and the pedestrian walkway, which is not in the current bridge. At either end of the bridge deck, expansion joints are also included. Rural areas are susceptible to extreme temperatures (relative to urban areas). This causes creep and shrinkage in the bridge deck. Expansions joints are used to avoid cracking. The bridge deck is a composite steel and concrete girder. For ease of construction, this is designed to be prefabricated offsite, and connected in-situ. The 55m bridge deck will have five 11m segments. Bridge Construction Assumptions 1. Sufficient workers would be away in December posing a resourcing issue. 2. There is sufficient site access for cranes and other machinery during construction. 3. The roads are wide enough to transport 11m long girders to the location. 4. Construction will not impact the integrity of the existing foundations and abutments. 5. During construction, partial highway lane closure beneath the bridge is approved by council. Construction Methodology Assuming the current bridge and highway are state owned roads, a detour is set up. It is not possible to construct the bridge in 1 week. Hence, bridge traffic could be detoured via council roads (often shortest detour, but a fee needs to be paid to the council), or via state roads (often longer, but less cost for the state). A longer detour, whilst maybe cheaper, would have a worser economic impact due to delay and emissions. Council roads are chosen. Once the detour is set up, the current bridge’s super structure can be demolished and removed from site. This can be performed over a 9-day period, Saturday to Sunday of the next week. To decrease demolition time, all lanes in the highway beneath will be closed. Considering the off-peak month April, this will get council approval. Then, the partial highway closure takes place, and the arch is built first in-situ, over the existing pier foundations. Once done, and cables are connected, the bridge segments are trucked to site. As segments are bolted laterally, cables are connected and anchored to deck so decks are free hanging. Process is continued from either end simultaneously as segments meet in the middle and bolted to completion. This causes lane closure both ways but decreases construction time drastically. Construction will take place at night to reduce disruptions. References De Los Santos, E 2021, Design Considerations for Arch Bridges, www.midasbridge.com, https://www.midasbridge.com/en/blog/bridgeinsight/design-considerations-for-arch-bridges